Considerations in optimizing CMB polarization experiments to constrain inflationaryphysics

Abstract

We quantify the limiting factors in optimizing current-technology cosmic microwavebackground (CMB) polarization experiments to learn about inflationary physics. Weconsider space-based, balloon-borne and ground-based experiments. We find thatforeground contamination and residuals from foreground subtraction are ultimatelythe limiting factors in detecting a primordial gravity wave signal. For full-skyspace-based experiments, these factors hinder the detection of tensor-to-scalar ratios ofr < 10− 3 on large scales, while for ground-based experiments these factors impede the ability toapply delensing techniques. We consider ground-based/balloon-borne experiments ofcurrently planned or proposed designs and find that it is possible for a value ofr = 0.01 to be measuredat the ∼3 σ level. A small space-based CMB polarization experiment, withcurrent detector technology and full sky coverage, can detectr ∼ 1 × 10− 3 atthe ∼ 3 σ level, but a markedly improved knowledge of polarized foregrounds is needed. We advocateusing as wide a frequency coverage as possible in order to carry out foreground subtractionat the per cent level, which is necessary to measure such a small primordial tensoramplitude.To produce a clearly detectable(> 3 σ) tensor component in a realistic CMB experiment, inflation must either involve large fieldvariations, or multi-field/hybrid models. Hybrid models can be easily distinguished from large field modelsdue to their blue scalar spectral index. Therefore, an observation of a tensor/scalar ratio andn < 1 in future experiments with the characteristics considered here may be an indication thatinflation is being driven by some physics in which the inflaton cannot be described as afundamental field.